I used to belong to the Columbia House Classical Music Club, which had the same problem. If I didn't return the card in time (which was not postage prepaid, if I recall correctly), then I received the selection of the month automatically. I could always return the selection of the month if I didn't want it, but it cost postage. Sometimes there seemed to be insufficient time between receiving the card and receiving the selection, particularly if I was out of town when the card arrived.
I finally did beat the system, though. They stopped carrying LP's, and kept sending me letters that I had to choose whether I wanted the selections sent in Cassette or CD format, to which I never responded. As a result, I received a card every month telling me what the selection of the month was, and reminding me that I had not specified what format I wanted it in. If I wanted it I could order it, if not I didn't have to do anything, they never sent me any unwanted music. I also did receive one free selection for every one I bought.
I finally left the club when I married and moved across the river. But I did get quite a large LP collection while I was in there.
Quod scripsi, scripsi.